Multi-layer membrane and the use thereof for the separation of liquid mixtures according to the pervaporation process

ABSTRACT

A multi-layer membrane having a porous backing layer of polyacrylonitrile, polysulfone or the like, and an active separating layer of polyvinyl alcohol or cellulose acetate. The membrane is particularly suitable or the separation of water-alcohol mixtures according to the pervaporation process.

BACKGROUND OF THE INVENTION

This is a divisional application based on U.S. Ser. No. 07/197,055,filed Apr. 27, 1988, now abandoned which is a divisional applicationbased on U.S. Ser. No. 06/778,741, filed Sep. 23, 1985 (now U.S. Pat.No. 4,755,299), which is a continuation of Ser. No. 06/499,847, filedJun. 1, 1983 (now abandoned).

It is known in the art that liquid mixtures can be separated in that theliquid mixture is placed into contact with one side of a suitablepolymeric membrane while a vacuum is applied to the other side of thepolymeric membrane or an inert gas stream is guided past the same. Someof the components of the liquid mixture that permeate more rapidlythrough the polymeric membrane are continuously removed in vaporizedcondition from the side of the polymeric membrane in communication withthe gaseous phase either by evacuation or by the inert gas stream.Non-permeating components are retained in the liquid phase accumulatingtherein until the desired degree of separation of components of a rapidand of a poor permeation and the desired degree of purity of theretained components of an inferior permeation, respectively, has beenattained.

What is particularly noteworthy regarding this process, called liquidpermeation or pervaporation and used for the separation of gas mixturesby means of membranes, is the fact that it also permits decomposition ofsuch liquid mixtures into their components that cannot be separated bysimple distillation because they either form azeotropes or the boilingpoints of the components are so close as to prevent an effective andeconomical separation. For, in pervaporation, it is no longer thepartial vapor pressure of the components above the liquid thatdetermines the composition of the mixture in the gaseous phase, butrather the different permeability of the membrane and, hence, theselectivity thereof for the various components in the liquid mixture. Onlaboratory scale, for example, mixtures of benzene/cyclohexane andisopropanol/water could be separated beyond the respective azeotropicmixtures by means of pervaporation. Similarly, it was possible toseparate the xylene isomers o-xylene (boiling point 144.4° C.), m-xylene(boiling point 139.1° C.) and p-xylene (boiling point 138.3° C.) bypervaporation in laboratory.

A special position is occupied by the mixtures of the simpleoxygen-containing organic compounds, e.g. of the simple alcohols,ketones, ethers, aldehydes and acids, with water. On the one hand, thesecompounds, frequently, are technically important substances required inlarge scale for the most various applications in dry and anhydrouscondition; conversely, these compounds, in general, completely orlargely are miscible with water forming azeotropic mixtures with waterso that the separation and recovery of the anhydrous organic substancesenvolve substantial expenditure. Many attempts have therefore been madeto use pervaporation processes for the separation of such mixtures;however, the efforts so far taken have never exceeded the stage oflaboratory tests. Admittedly, the prior art membranes of cellulosediacetate and triacetate that are also employed otherwise, e.g., inreversing osmosis, for some purposes have an adequate mechanicalstability and a satisfactory flow of permeate; However, the selectivitythereof does not yet permit a large-scale industrial use forpervaporation. The present invention is now concerned with a multi-layermembrane having a non-porous separating layer from a first polymer and abacking layer from a second polymer, which is characterized in that theseparating layer is comprised of polyvinyl alcohol or cellulose acetate.

SUMMARY OF THE INVENTION

The membrane of the invention is suitable for the separation of liquidmixtures by means of liquid permeation or pervaporation, especially forthe separation of water from its mixtures with oxygen-containing,organic liquids, such as simple alcohols, ethers, ketones, aldehydes oracids. Moreover, the membrane of the invention is also suitable for theseparation of gas mixtures.

The membranes of the invention, owing to their multi-layer structures,on the one hand, have an excellent mechanical stability and, on theother hand, the separating layer can be applied to the mechanicallystable backing layer. With a thickness which is sufficiently thin toprovide selectivity but also to permit industrial use of such membranes.In technical usage, the multi-layer membranes are also designated ascompound or composite membranes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In accordance with the present invention, the separating layer of themembrane is comprised of polyvinyl alcohol or cellulose acetate.Polyvinyl alcohol is usefully obtained by saponification of polyvinylacetate. Preferably, a polyvinyl alcohol is used that has a highsaponification degree, e.g., a saponification degree in excess of 98 or99 percent. The molecular weights are uncritical, if only film formationand membrane formation, respectively, are safeguarded. Usual molecularweights are within the range of between 15.000 and 200.000, e.g. between70.000 and 120.000 (Daltons). Suitable products are commerciallyavailable. The cellulose acetates, in the first place, are cellulosediacetate and triacetate having the characteristics normally used in theproduction of membranes. Separating layers from polyvinyl alcohol arethe preferred ones as they yield substantially improved results both asto selectivity and permeate flow.

In the separating layers it is necessary (as it is with the porousbacking layers) that the polymers used in the present invention not beremoved or attacked either by water or by the solvents to be separated.

The use of polyvinyl alcohol as a separating layer exhibits quite anumber of characteristics. Polyvinyl alcohol is easily soluble in waterfor which reason it can in simple manner be applied from an aqueoussolution, whereas polyvinyl alcohol is insoluble in all simple organicsolvents. Polyvinyl alcohol, chemically and thermally, is stable;polyvinyl alcohol layers can be after-treated by cross-linkage to beextent that, in the long run, they are also insoluble in hot watershowing only a swelling with water that can be adjusted by the type ofthe cross-linking reaction. Very thin, firmly adhering separating layersof polyvinyl alcohol that have an adequately high permeate flow can beapplied to suitable backing layers. Owing to the conditions ofmanufacture, the properties of the polyvinyl alcohol separating layersare widely variable so that the pervaporation membranes producedthereby, with respect to selectivity and permeate flow can be optimallyadapted to the respective problem of separation.

Insolubility of the polyvinyl alcohol in water is caused bycross-linkage. Preferably, cross-linking is performed by etherification,esterification or acetalization, or by a combined use of the saidprocesses. Examples are esterification with dicarboxylic acids,preferably those that, in addition, contain hydroxyl- and/orketo-groups; etherification under catalytic exposure to acids, or bymeans of dihalogen compounds, such as 1,3-dichloroacetone or1,3-dichloroisopropanol, or acetalization by means of aldehydes ordialdehydes. Based on ethanol/water mixtures, esterification, ingeneral, results in an increased selectivity and acetalization leads toan increase in the rate of permeate flow while the influence ofetherification on selectivity and permeate flow rate is less pronounced.The afore-going, in analogy, applies to other hydrous mixtures. Theeffects of a number of different cross-links are revealed by theexamples.

The separating layer of polyvinyl alcohol or cellulose acetate isrequired to be non-porous and free of defects (free of holes). Thethickness of the separation layer, in general, amounts to between 0.05and 10 μm, preferably to between 0.1 and 5 μm, with layer thickness ofabout 1 to 2 μm, in practice, having provided to be particularlysuitable.

The non-porous separating layer, dissolved in a suitable solvent, isdirectly applied to the porous backing layer. The concentration in thisrespect is non-critical, however, hardly will it be possible to attainsufficiently thin layers by excessively viscous solutions. The preferredsolvent for polyvinyl alcohol is water, for cellulose triacetate it ischloroform or trichloroethylene.

Generally, all substances suitable for use as ultrafiltration membranescan be considered as porous backing layers for the multi-layer membranesof the invention. Owing to the desired thermal stability and theunsensitivity to the solvent mixtures to be separated, porous backinglayers of polyacrylonitrile (PAN), polysulfone (PS) and hydrolyzed orsaponified cellulose acetates will be preferred. Preferably, the porousbacking layer has a very close distribution of pore radii with anaverage pore radius such that the macromolecules of the polymer used forthe separating layer of the membrane, preferably polyvinyl alcohol,cannot penetrate into the pores of the backing layer but are ratherretained on the surface. In this manner, very uniform, thin andefficient separating layers can be applied to the porous backing layer.The adjustment of the pore radii and of the distribution of the poreradii, on the one hand, can be effected by corresponding conditions inthe preparation of the porous backing layer as such, or it can beeffected by applying an intermediate layer to a less suitable porousbacking layer, in which case, the separating layer is applied to theintermediate layer (see e.g. examples 5 and 6 ). When using celluloseacetate for the separating layer, pore distribution and average poreradius of the backing layer are less critical, as the molecular weightof cellulose acetate is substantially higher than that of polyvinylalcohol and, beyond that, also a different molecular configurationprevails. Finally, solutions from cellulose acetate in organic solventsare of a higher viscosity than corresponding aqueous polyvinyl alcoholsolutions. For all these reasons, the use of cellulose acetate as aseparating layer is less sensitive regarding the pore distribution andthe pore radius of the porous backing layer.

The thickness of the porous backing layer is not critical, if only anadequate mechanical strength of the multi-layer membrane is safeguarded.The thickness of the porous backing layer amounts to e.g. 20,50 or 100μm, or more.

In a preferred embodiment, the porous backing layer in the inventivemembrane is applied to a fleece or to a fabric serving as a carryinglayer. As are the other layers, the carrier layer, preferably, isresistant to temperature and chemicals. The carrier layer, on the layerside, preferably, is smooth to avoid a damage to the porous backinglayer. Polyesters are the preferred ones; cellulose layers, in general,are not sufficiently smooth. The polyamides actually suitable, ingeneral are less preferred owing to the thermal resistance thereof lowerthan that of polyesters and owing to their lower solvent strength. Thethickness of the carrier layer is not critical either; in practice,thicknesses of between 50 and 150 μm, e.g. approximately 100 μm, haveproved to be particularly suitable.

Application and distribution of the polymeric solutions forming theporous backing layer and the non-porous separating layer, generally, areperformed in a manner that the polymeric solutions, by way of a knifeblade or glass rod, are distributed over and swept clear of thecorresponding layer. For this, the layer thicknesses are adjusted inaccordance with the desired layer thickness. In addition to thatprocess, in practice, especially for less viscous polymeric solutions,also a process has proved to be suitable that in the American-languageliterature has been described as the "meniscus coating" or "dipcoating". The carrier material to be laminated, with the side to becoated is drawn downwardly over a roll just touching the surface of thepolymeric solution to be applied. A meniscus is then formed between theliquid surface and the carrier material, with the surface of the carriermaterial being wetted by the polymeric solution. In some cases, it ispreferred to improve the wettability of the carrier material by theaddition of wetting agents. By varying the viscosity of the polymericsolution, the drawing speed, the carrier material and of the drippingtime of the polymeric solution, the thickness of the so applied layer ofthe polymeric solution can be widely varied and reproducibly adjusted.

The permeate flow in kg/h.m² under the test conditions of thetemperature, the composition of the mixture to be separated and of thepressure on the permeate side, and the selectivity B of the membraneunder the said conditions serve for characterizing of pervaporationmembranes. B is a non-dimensional figure representing the concentrationratio of the binary mixture in the permeate divided by the concentrationratio in the feed. ##EQU1##

The volume of the permeate flow shows heavy dependence on thetemperature. While in all examples of embodiment the feed of the liquidmixture was performed under atmospheric pressure, the pressuresprevailing on the permeate side amounted to between 10 and 50 mbar. Inthat range the level of the permeate-sided pressure was without anynoteable influence on the permeate flow and the selectivity of themembranes.

The invention will now be described with reference to the drawings,wherein

FIG. 1 is a cross-sectional view of a preferred embodiment of amulti-layer membrane of the invention, and

FIG. 2 is a cross-sectional view of another preferred embodiment of amembrane according to the invention.

According to FIG. 1, a multi-layer membrane 1 is comprised of apolyester fleece carrier layer 2 having a thickness of 120 μm. Providedthereon is a porous backing layer 3 of polyacrylonitrile having athickness of 50 μm. The separating layer 4 is comprised of polyvinylalcohol cross-linked with maleic acid and has a thickness of 1,2 μm. Themanufacture of that multi-layer membrane will be described in example 1.

According to FIG. 2, a multi-layer membrane 21 is comprised of apolyester fleece carrier layer 22 having a thickness of 120 μm. Providedthereon is a porous backing layer 23 of polyacrylonitrile having a layerthickness of 50 μm. Provided thereon is a porous intermediate layer(another backing layer) 24 of saponified cellulose triacetate having athickness of 50 μm. A non-porous separating layer 25 of polyvinylalcohol cross-linked with maleic acid has a layer thickness of under 1μm. The production of that multi-layer membrane will be described inexamples 5 and 6.

The examples will explain the invention.

The carrier layer as used in the examples is a polyester fleece having alayer thickness of about 120 μm.

The polyvinyl alcohol (PVA) as used is a commercially available producthaving a degree of saponification of at least 99 percent and an averagemolecular weight of 115.000 (Daltons).

EXAMPLE 1

A 15% solution of polyacrylonitrile (PAN) in dimethyl formamide (DMF) bymeans of a knife blade is applied as a layer of a 50 μm thickness to apolyester fleece forming a carrier layer, and is precipitated accordingto the phase inversion process at a temperature of 8° C. The resultingporous membrane, at a pressure differences of 4 bar, shows a clear waterdischarge rate of 150 l/h.m² and a retention capacity of more than 99.5percent for a 1% solution PVA in water.

Subsequently applied to that PAN membrane is a solution of 7 percent byweight PVA in water added to which are 0.05 mole maleic acid per molemonomeric unit PVA. After drying, hardening and cross-linking of the PVAseparating layer at 150° C., the PVA separating layer will no longer besoluble either in boiling water. Tests with water/alcohol mixtures, at afeed temperature of 80° C. and a water/alcohol ratio in the feed of 1/4resulted in a selectivity of B=1400 and a rate of permeate flow of 0.04kg/h.m².

Under otherwise identical conditions except for a water/alcohol ratio inthe feed of 5/95, the selectivity was still 9500 at a rate of permeateflow of 0.01 kg/h.m².

EXAMPLE 2

Example 1 is repeated, except for that a reduced PVA concentration of 5%is used in the preparation of the PVA separating layer. Under theconditions as specified, the following values are measured on the finalmembrane:

Feed 12 percent by weight water, 88 percent by weight ethanol, feedtemperature 80° C., selectivity 250, rate of permeate flow 0.05 kg/h.m².

Feed 20 percent by weight water, 80 percent by weight isopropanol, feedtemperature 45° C., selectivity 250, rate of permeate flow 0.3 kg/h.m².

Feed 20 percent by weight water, 80 percent by weight acetone, feedtemperature 60° C., selectivity 100, rate of permeate flow 0.25 kg/h.m².

EXAMPLE 3

A PAN membrane produced in accordance with example 1, is coated with anaqueous solution of the following composition: PVA 5 percent by weight;formaldehyde 1 mole per mole PVA monomeric unit; hydrochloric acid 1mole per mole PVA monomeric unit.

After hardening at 155° C., the following separating performances aredetermined for ethanol/water mixtures at a temperature of 70° C.:

Feed 80 percent by weight ethanol, selectivity 30, rate of permeate flow1.5 kg/h.m².

Feed 90 percent by weight ethanol, selectivity 50, rate of permeate flow1.0 kg/h.m².

Feed 99 percent by weight ethanol, selectivity 30, rate of permeate flow0.25 kg/h.m².

EXAMPLE 4

An 18% solution of polysulfone (see Condensed Chemical Dictionary, 8thedition, 1971, p. 712) in DMF, in accordance with the description inexample 1, is applied to polyester fleece as a carrier layer and isprecipitated in water at a temperature of 8° C. by phase inversion.Applied to the so formed porous backing layer is an aqueous solution of6 percent by weight PVA containing 0.05 mole fumaric acid per molemonomeric unit PVA and is hardened at 150° C. At a temperature of 80° C.and at a feed concentration of 80 percent by weight ethanol in water, aselectivity of 350 and a permeate flow of 0.2 kg/h.m² were measured.

EXAMPLE 5

A 15% solution of PAN in DMF, according to example 1 is applied to apolyester fleece forming the carrier layer and is precipitated in waterat a temperature of 15° C. The so obtained porous membrane shows a clearwater discharge of more than 150 l/h.m², a retention capacity for a 1percent PVA solution of 90 percent at an average molecular weight of thePVA of 11500, and of 50 percent at an average molecular weight of thePVA of 72000. After drying, a 1% solution of cellulose triacetate inanhydrous chloroform is applied to the said membrane by "dip-coating",and the solvent is evaporated under the exclusion of moisture. At atemperature of 80° C., the said membrance, at a feed concentration of 80percent ethanol in water, showed a selectivity of 10 at a permeate rateof flow of 2 kg/h.m².

EXAMPLE 6

The membrane provided with a separating layer of cellulose triacetateobtained according to example 5 was exposed to an aqueous ammoniasolution of a pH-value of 10.5 until complete saponification of thecellulose triacetate. Now the said porous membrane, for PVA of amolecular weight of 115000, showed a retention capacity of in excess of99.5 percent and, for PVA of a molecular weight of 72000, showed aretention capacity of 98 percent. Coated with a 3 percent PVA solutioncontaining maleic acid as the cross-linking agent, at a feed temperatureof 78° C. and at an ethanol concentration of 80 percent, a selectivityof 250 was obtained at a permeate flow rate of 0.5 kg/h.m².

What is claimed is:
 1. A composite membrane consisting essentially of(a) a non-porous separating layer having a thickness between 0.05 and 10μm, said separating layer being useful for separating an aqueous liquidmixture comprising at least one organic liquid, said separating layerbeing comprised of crosslinked polyvinyl alcohol, (b) at least oneintegral porous backing layer comprised of a second polymer, and (c) atleast one integral porous backing layer comprised of a third polymer,wherein said cross-linked polyvinyl alcohol is disposed on the surfaceof said porous backing layer (b) so that said separating and backinglayers are distinct and wherein said composite membrane is a flat sheetmembrane constructed and arranged for the separation of mixtures in apervaporation process.
 2. A membrane according to claim 1, wherein saidnon-porous layer has a selectivity B value of at least 30 for a mixturecomprising 80 wt % ethanol and 20 wt % water, relative to the totalweight of said mixture.
 3. A membrane according to claim 1, furthercomprising a carrier layer (d), said carrier layer comprising a fleeceor fabric.
 4. A membrane according to claim 3, wherein said thirdpolymer is a polyacrylonitrile.
 5. A membrane according to claim 3,wherein said third polymer is a polysulfone.
 6. A membrane according toclaim 3, wherein said carrier layer has a thickness of between 50 and150 μm and is comprised of a polyester.
 7. A membrane according to claim1, wherein said third polymer is a polyacrylonitrile.
 8. A membraneaccording to claim 1, wherein said third polymer is a polysulfone.
 9. Amembrane according to claim 1, wherein said porous backing layer (b) hasa close distribution of pore radii and has an average pore radius suchthat polyvinyl alcohol molecules from said non-porous separating layerdo not penetrate into the pores of said porous backing layer (b).
 10. Amembrane according to claim 9, wherein said second polymer is asaponified cellulose acetate.
 11. A membrane according to claim 10,wherein said second polymer is saponified cellulose triacetate.
 12. Amembrane according to claim 11, said membrane comprising two porousbacking layers, at least one of said backing layers being comprised ofsaponified cellulose triacetate.
 13. A membrane according to claim 1,wherein said cross-linked polyvinyl alcohol is the product of a processcomprising (i) a step selected from the group consisting ofetherification, esterification with a monobasic or a dibasic acid,acetalization with a monofunctional or difunctional aldehyde, and acombination thereof, wherein heat is applied during step (i).
 14. Amembrane according to claim 1, wherein said cross-linked polyvinylalcohol is cross-linked with maleic acid.
 15. A membrane according toclaim 1, said membrane accommodates a permeate flow of at least 1.5kg/h.m² for said mixture.
 16. A membrane according to claim 1, whereinsaid non-porous layer has a thickness of between 0.1 and 5 μm.
 17. Amembrane according to claim 16, wherein said non-porous separating layerhas a thickness between about 1 and 2 μm.
 18. A membrane according toclaim 1, wherein said polyvinyl alcohol has a saponification degree inexcess of 98 percent.
 19. A membrane according to claim 1 which isproduced by dissolving said polyvinyl alcohol in a solvent to form asolution, and applying the solution to backing layer (b).
 20. A membraneaccording to claim 1, wherein said backing layers have a combined has athickness of more than 20 μm.